Signal distortion in atomic force microscopy photodetector.

The frequency-dependent complex impedance of an atomic force microscope photodetector is measured. The inverse problem is solved obtaining the voltage that would have been collected with a hypothetical, perfectly flat-frequency-response photodetector from the experimentally available voltage. This information is used to study the distortion that the true input signal undergoes as it passes through the photodetector on the way to becoming the experimentally measured output signal. It is found that signals with features of interest shorter than 10 μs render noticeable differences between the true and measured raw voltages and forces. Signals with features shorter than 1 μs produce experimentally measured force curves that deviate substantially from the true force curves. A method is proposed for correcting the measured raw voltage signal.